Not applicable.
1. Field of the Invention
This invention relates generally to optical amplifiers and more particularly to a system and method for auto-shutdown of distributed Raman amplifiers in optical communications systems
2. Background of the Invention
As is known in the art, distributed Raman amplifiers in optical communications systems function by injecting a high-power optical beam into the transmission fiber. Energy is transferred from the Raman pump laser to the signals as they propagate in the fiber. Stimulated Raman scattering is an intensity dependent process, hence the optical power requirement of the Raman pump laser increases as the optical gain and bandwidth of the amplifier are increased.
The Raman pump laser is injected into the transmission fiber and the communication system is considered as a closed system such that during normal operation there is no risk of human exposure to dangerous optical power levels. However, there can exist a significant safety hazard and risk of exposure to high optical powers from the amplifier if there is a break in the transmission fiber or the Raman pump laser is disconnected from the transmission fiber while the unit is active.
In order to meet US and European safety regulations, as well as to significantly reduce any exposure risk, it would be desirable if it was possible to detect if amplifier was disconnected from the fiber span or if there was a break in the span, and automatically reduce it""s output power level to an acceptable safety limit.
It would, therefore, be desirable to provide a system and technique for detecting if an optical module has been disconnected from a fiber span or if there has been a break in the span, and for automatically reducing the output signal level of optical module such that the output signal level is within an acceptable safety limit.
It would also be desirable to provide a system and technique for automatically resetting a Raman pump unit once the source of the optical leak has been located and addressed.
In accordance with the present invention, a technique for detecting if an optical module has been disconnected from the fiber span or if there has been a break in the span, and for automatically reducing the output of an amplifier such that the output is within an acceptable safety limit is provided.
In one embodiment, for a counter-propagating distributed Raman amplifier, where the signals and Raman pumps are travelling in opposite several criteria can be used. For an amplifier providing gain to the C- and L signal bands, the a break in the transmission line can be detected by: (1) a loss of signals in both the C-band and L-band or (2) a high amount of Raman pump light backreflected in the Raman pump module.
The loss of signal can be determined by measuring the optical power as tapped from the main transmission path. It is preferred that the C and L-bands first be demultiplexed before detecting the signal power. A loss of signal in a given band would be determined if the signal power level falls below a set minimal threshold level. To determine a break anywhere in the fiber span, this level should be set at a power above the Raman generated ASE at measured at the Raman pump module Lower power levels will restrict the detection of a fiber break to a limited section of the transmission span. This may be acceptable, as this will allow monitoring of a portion of the fiber span where the Raman pump power is greatest.
The high backreflection criteria is based on the fact that when the Raman pump module is disconnected from the transmission span, xcx9c4% of the pump light will be reflected back into the module if a flat polished connector is used When the module is connected to the span, typically less than 0.2% of the light is backreflected into the module due to Rayleigh scattering in the fiber. The power threshold level for shutdown should be set between 4% and 0.5% of the nominal operating Raman pump power level.
If either criteria 1 or 2 is detected, the module should automatically turn off in a time period short enough such that the maximum permitted optical exposure does not exceed ANSI safety limits.
If the Raman pump unit is on and there is any detectable signal power in either the C-band, L-band, or the supervisory channel, the Raman pump unit should override the shutdown circuitry and reset to the operating power level for a time period not to exceed the maximum permitted exposure. At the end of the time period, the shutdown circuitry should be re-enabled.